1. Field of the Invention
This invention refers to a tyre for vehicle wheels and a bead core included in this tyre. More in particularly, the invention refers to a tyre for high or very high performance comprising a single-wire bead core with a hexagonal section.
2. Description of the Related Art
A tyre, in general, comprises a carcass formed by at least one carcass ply shaped in a toric form, the ends of which are anchored around substantially inextensible annular elements, called bead cores, a tread and a belt structure located between the carcass and the tread. The portion of tyre which comprises the bead core is known as a bead and its function is to anchor the tyre on its respective rim. In a traditional structure, the bead usually comprises an appropriate rubber fillet with a substantially triangular section, known as bead filler.
Several types of bead cores and different bead structures are known in the state of the art.
A bead core of a known type has a transverse regular quadrangular section. Here, and in the description which follows, “transverse section” is intended as a section made by means of a plane containing the axis of rotation of the tyre. A regular quadrangular bead core can be formed, for example, starting from a rubber-coated strip, comprising five wires, substantially directed parallel to the axis of the strip itself. Here, and in the description which follows, the term “wire” is intended as a metal wire or a metal cord. The bead core is formed by winding the rubber-coated strip in a spiral, in such a way as to form five layers superimposed in a radial direction, each composed of five windings (or turns). This configuration is referred to as “5×5”.
Here, and in the description which follows, “radial direction” is intended as a direction perpendicular to the axis of rotation of the tyre.
An example of a bead core with a regular quadrangular section is described in the patent GB 1,438,441, in the name of the same Applicant. Bead cores with a regular quadrangular section are currently used in the Applicant's tyres for high or very high performance.
The Applicant has discovered that in a bead core with a regular transverse quadrangular section, the distribution of the stress forces in the windings composing the various constructive layers of the bead core is not optimised. In particular, as will be shown better in the detailed description which follows, by means of a simulation of assembly of the tyre on a rim, the Applicant found that in the bead core with a regular quadrangular section, the stress to which the windings are subjected drops rapidly and progressively in the layers radially external to the first. This distribution of the stress values is not optimal. In fact, as the number of windings is the same in all the layers in the bead core with a regular quadrangular section, there is a waste of material (due to the high number of windings) in the radially outermost portion where, instead, the stress value is found to be very low.
The Applicant therefore tackled the technical problem of obtaining a bead core with a better distribution of the forces of stress in the windings among the various layers.
At the same time, in the tyre's working conditions, and at high speeds in particular, the bead core should have guaranteed excellent features of compactness and indeformability of the section, as well as steady support of the bead on the corresponding seat of the rim.
The Applicant perceived that the problem of the unbalancing of the stresses in the windings of the bead core could be solved by modifying the section of the bead core itself, reducing the number of windings of the radially outermost layers of the bead core.
Bead cores with sections other than a quadrangular one are known in the state of the art.
For example, a bead core is known which is formed starting from a single rubber-coated wire, with a regular transverse hexagonal section. Here, and in the description which follows, “hexagonal section” is intended as a section of which the profile can be represented by means of a polygon with six sides. The hexagonal section is regular, this intending to mean that it is symmetrical both with respect to a radial direction and with respect to an axial direction. Here, and in the description which follows, axial direction is intended as a direction parallel to the axis of rotation of the tyre. For example, a regular hexagonal bead core can be formed by 19 windings arranged in a 3-4-5-4-3 configuration. This set of numbers indicates that the single wire has been wound in such a way that it first of all constitutes three turns arranged axially side by side to form the first layer, then four turns arranged axially side by side to form the second layer, radially superimposed on those of the first layer, then five turns arranged axially side by side to form the third layer, radially superimposed on those of the second layer, then four turns again arranged axially side by side to form the fourth layer, radially superimposed on those of the third layer, and then three turns again arranged axially side by side to form the fifth layer, radially superimposed on those of the fourth layer. The layers have been numbered in accordance with their position in the radial direction: here, and in the description which follows, the first layer is intended as the radially innermost layer. Furthermore, below in the description, the notation “single-wire bead core” will be used to indicate a bead core formed by a plurality of windings of a single wire.
Examples of single-wire bead cores with a hexagonal section are given in the patent JP 02-296509 in the name of Toyo Tire & Rubber.
Other patents describe single-wire bead cores with different sections. Among these, the patents U.S. Pat. No. 5,058,649 and U.S. Pat. No. 5,524,688 in the name of Goodyear Tire & Rubber describe a single-wire bead core with a pentagonal section incorporated in a bead not comprising the above-mentioned bead filler.
In an alternative to the single-wire bead cores, bead cores described as “spiral” are known in the state of the art. This type of bead core is made up of a central core, formed, for example, starting from a single rubber-coated wire, around which other wires are wound in spirals in such a way as to form an external sheath. An example of a spiral bead core is given in the patent U.S. Pat. No. 1,437,013 in the name of International Bead Wire Company, which describes a bead core with a central core having a triangular section formed by a single wire around which a plurality of spiral windings of another wire are wound, forming the external sheath.
After being formed in this way, the bead core is extended radially in such a way as to give the external sheath, which initially had a circular section, a section determined by the section of the central triangular core, or a hexagonal section.
Patent '013 states that the transverse section of the bead core obtained in this way is particularly suitable for tyres for vehicles for heavy-goods transport.